In general, a fluororubber is obtained by blending a crosslinking agent or a crosslinking accelerator with a reinforcing inorganic filler such as carbon black and silica, and it is crosslinked and molded to be used as a sealing material. Also, a molding material for a stopper which is built into a hard disc drive is obtainable by blending carbon black as the reinforcing inorganic filler as disclosed in Patent Documents 1 and 2, for example. However, due to a reduction (narrowing) of a gap (clearance) between a disc and a head involved in a recent increase in hard disc drive capacity, there has been a problem that a failure in writing or reading occurs when the filler contained in the rubber composition is separated from the rubber composition.
As a countermeasure for the problem, use of a rubber composition which can be crosslinked without using the reinforcing inorganic filler is contemplated, but such a countermeasure entails problems such as deterioration in workability of the rubber composition free from the reinforcing inorganic filler during roll mixing and a failure in attaining desired hardness of a crosslinked and molded product.
Meanwhile, use of PTFE as a filler which enables to finely adjust hardness and to ensure strength is contemplated. Patent Document 3 describes that a crosslinked and molded article of a fluororubber composition containing 1 to 100 parts by weight of PTFE, preferably PTFE in the form of a fine powder, relative to 100 parts by weight of a fluororubber has excellent compression set and chemical resistance and is capable of attaining shore A hardness of about 55 to 90.
In the case where the PTFE content is increased for the purpose of attaining shore A hardness of 70 or more, the polymer becomes excessively hard to deteriorate kneading workability, resulting in addition of a step of preliminarily heating the polymer or the like and in a cost increase for kneading using a high-power kneader or the like. Therefore, it can be said that the upper limit of shore A hardness is about 70 in the case of improving hardness only by the blending of PTFE.